Interaction of acrylic-maleic copolymers with H +, Na +, Mg 2+ and Ca 2+: Thermodynamic parameters and their dependence on medium

Abstract

Thermodynamic parameters for the interaction of two acrylic-maleic copolymers (MW = 3 and 70 kDa) with H +, Na +, Mg 2+ and Ca 2+ were obtained from potentiometric and calorimetric measurements. Apparent protonation constants were expressed as a function of α (dissociation degree) using the three parameter model proposed by Högfeldt and the formation constants of Na +, Mg 2+ and Ca 2+ complexes were calculated from the differences in protonation constants in the different ionic media. The salt effects on protonation were interpreted in terms of both activity coefficient variations and cation complex formation. Enthalpy changes for protonation, obtained by direct calorimetry (always quite low, the stabilization being mainly of entropic nature), were also expressed as a function of α and I using empirical equations. The formation constants obtained for alkali and alkaline earth metal complexes are quite low for Na + species (depending on α and on ionic strength, we always found K < 10 L mol −1), and fairly high for Mg 2+ and Ca 2+ species, ML and ML 2. Comparison is made with analogous results obtained for polyacrylic and polymethacrylic acids.